The present invention relates to composites having a diamond surface and more particularly to composite fibers having a diamond coating as a surface. The present invention also relates to methods of growing of large diamond deposits over large areas and to the growing of polycrystalline films of diamond of controlled orientation and crystallite size.
It is, of course, well known that diamond crystals can be grown or produced "synthetically" by closely controlling the requisite chemical and physical conditions. Such crystals, however, were relatively small in size and weight (i.e. crystals as opposed to a contiguous film). No one has ever attempted to continuously deposit diamond on a fiber substrate to produce a composite fiber.
Typically, the early diamond deposition processes were carried out under extremely high pressures and temperatures, e.g. about 60,000 atmospheres and 1700.degree. C. These conditions were obviously difficult and expensive to maintain and more recent efforts have been directed at the production of diamond crystals under low pressures, i.e. below atmospheric pressure and at more moderate temperatures.
The use of low pressure techniques requires careful control of other conditions, since below atmospheric pressure diamond is the unstable form of carbon, and graphite is the stable form. Thermodynamically, a stable solid should form preferentially over an unstable solid; however, it has been well-established that diamonds can be grown from energetically activated gases at low pressures in spite of the theoretical thermodynamic instability.
Typical conditions at which such diamonds are grown are a total pressure of about twenty (20) Torr, gas composition of one volume percent methane and hydrogen, and a substrate temperature of about 900.degree. C. Typically, energy is added to the gas by a number of means including use of a heated filament, e.g. tungsten, or a microwave discharge. It is generally believed that the energy added to the gas aids growth of the diamond crystal by fragmenting the hydrocarbon molecules, e.g. methane, into a more chemically reactive species such as methyl radicals, and it is also believed to cause the dissociation of the molecular form of hydrogen, H.sub.2, to atomic hydrogen, which is also believed to enhance the growth process.
Because graphite is the thermodynamically stable phase of carbon, it was heretofore believed that it, and related graphite materials, should be rigorously excluded from the diamond growth chamber. In fact, it was believed that one of the primary functions of the atomic hydrogen produced in the growth chamber was to remove all traces of graphitic forms of carbon. In order to promote the nucleation of polycrystalline diamond films, the conventional procedure was to treat the surface with diamond powder prior to the growth process. Recently, the work of Angus et al. (U.S. Ser. Nos. 878,717; 878,721; and, 878,255) disclosed techniques for depositing diamond crystals on a variety of substrates including graphite.